The most well known method for manufacturing hollow products up to the present time is hollow-molding. By this method, however, it is difficult to manufacture products with complicated geometries such as, for example, automotive intake manifold having a plurality of curved passages, because of the fact that the structure of the die which can be used in this method should not be that complicated.
On the other hand, there has been an attempt to manufacture hollow products with complicated geometries by injection molding out of heat resisting polymers or water resistant polymers such as PE, PP, PS, or the polymers which are made by adding to said heat resisting polymer fillers such as glass fiber, carbon fiber, carbon tarch, or calcium carbonate in order to reduce weight of the products. However, the above described obstructions in hollow-molding still remain. Therefore, there is no other method but making such products out of metallic materials such as aluminum by metal casting.
To this end, Belland AG of Switzerland suggest a method for manufacturing a plastic air intake manifold or the like using lost core technique in the Journal of Modern plastics International, November, 1990, where they cannot disclose the method with satisfaction.